Química Orgánica 2 · Efectos de los sustituyentes y orientación
Nitration of pyridine at C3
Condiciones: HNO3, H2SO4, 300 °C
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Pyridine attacks the nitronium ion at C3
The nitronium ion, O=N+=O, is formed from nitric acid by sulfuric acid (protonation of the N–OH, loss of water); it is drawn here already formed. Pyridine is very unreactive (the electronegative nitrogen withdraws density, and in acid much of it is protonated), so very harsh heating is needed. The C3=C4 π pair forms the C–N bond at C3.
Las flechas curvas
- The ring π pair forms the new C–N bond to the nitronium nitrogen.
- One N=O π pair moves onto oxygen so nitrogen does not exceed four bonds; the new group is a neutral nitro group.
Delocalize the charge without touching nitrogen
After C3 attack the positive charge is shared by C2, C4 and C6, all carbons. After C2 or C4 attack one contributor would put the positive charge on nitrogen with only six electrons, a very bad contributor, so C3 attack is least disfavoured.
Las flechas curvas
- The C5=C6 π pair shifts toward the cation, moving the positive charge to C6; at no point does the charge reach nitrogen.
Why C3 for pyridine?
Avoid a sextet nitrogen cation.
Hydrogen sulfate removes the ring proton
Hydrogen sulfate, the strongest base in mixed acid, removes the hydrogen on the sp3 ring carbon; the electrons flow back around the ring and aromaticity is restored.
Las flechas curvas
- A hydrogen sulfate oxygen lone pair takes the hydrogen on the sp3 ring carbon.
- The C–H bond pair becomes a ring π bond, starting to restore the aromatic sextet.
- A ring π pair shifts one bond along to make room for the new π bond.
3-nitropyridine
Pyridine gives 3-nitropyridine, in low yield, plus regenerated sulfuric acid.
Qué vas a practicar
Draw nitration of pyridine at C3 and show that its σ complex never places the positive charge on nitrogen.
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